In such a system, signal samples digitized by pulse-code modulation (PCM) constitute bit groups--known as bytes--which are interleaved by a multiplexer at one TDM terminal in respective time slots of a recurrent data-stream frame, these time slots being individually assigned to the several communication channels whose messages are to travel over a common transmission path to another TDM terminal for redistribution by a demultiplexer and subsequent decoding. The decoded signal amplitudes should, of course, conform as closely as possible to those of the original analog signals; thus, the maximum signal level at the point of origin should always be translated into one and the same bit group yielding again a maximum level upon reconversion to analog form. To test the fidelity of digitization, it is therefore customary to apply a low-frequency signal to predetermined amplitude to the input end of a given channel of a PCM/TDM system and to detect the amplitude of the reconstituted low-frequency signal at the output end of that channel, i.e. after sampling, encoding, multiplexing, transmission, demultiplexing and decoding. Since, however, an error in the equipment of the transmitting terminal may be compensated by a complementary error in the equipment of the receiving terminal, this mode of testing is not a dependable gauge for the performance of the multiplexer and associated circuit components (e.g. binary coder) per se.